MacOS – Reset Uniform Type Identifiers on Mac OS X

There is not a safe and advisable way to achieve what you are after because this function of the OS is controlled by the Launch Services database. Developers register the filetypes their applications support with the OS on first launch.

The best way to get this fixed is to petition the developer to add support for the file extension in a future version of their application.

With that said you have already found the only documented way that Apple offers for overriding the edicts of the Launch Services database. If you want to dig deeper and take the road less traveled supported here are a few pointers.

There is a .plist file that governs part of what Launch Services does located at: ~/Library/Preferences/com.apple.LaunchServices.plist

Apple notes that removing this plist can clear out unwanted Open With contextual menu options. But that's the opposite of what you're after.

It's a binary .plist but you could convert it to an xml version and experiment a little to see if you can come up with a recipe to achieve what you're after. This isn't advisable, it could cause harm, and it will most likely get overwritten by the OS at some point.

There is also a tool hidden a bit deeper in the OS called lsregister which could be helpful in exploring Launch Services's plumbing. Here is a helpful explanation of lsregister and its options.

Column previews in Finder are in fact provided by QuickLook, not Preview – as are the icons displaying the file contents, the CoverFlow icons and the QuickLook preview proper you invoke by pressing Space or Cmd+Y on a selected file.

QuickLook uses plugins to display file contents – a piece of code rendering the file contents and telling the QuickLook server it can do so (see the QuickLook Programming Guide if you want the technical details). You need plugins because files can be binary data – without knowledge of its structure, no representation of the file is possible – or textual data that needs processing – HTML for instance, which you’ll probably prefer to preview rendered, not as raw source. They also must be limited to specific, known file types (to be precise: Uniform Type Identifiers, or UTIs for short) for this exact reason – a rendering plugin for HTML will not give you a usable preview for a JPEG. OS X provides quite an impressive default collection of these plugins (located in /System/Library/QuickLook), and applications you install add their own (located in the application package itself, /Library/QuickLook or ~/Library/QuickLook). You can also add some yourself to those two last locations.

In the case of text files without an extension, the problem is OS X’s built-in plain-text QuickLook plugin is not assigned to these as they do not correspond to a UTI it handles. However, there is a 3rd party QuickLook plugin adding that functionality: QLStephen. Just download it and install it according to its installation instructions on GitHub and you should be good to go.